copycat of 50 Wyoming
I thought you could do this from the 9.3-74R. However, I would point out the 50 Wyoming, as a start for a 30-06 belted. Not the squeezed down 240 Weatherby, but a full sized 30-06, with a belt and a slightly larger rim, than the parent case. You might even order it longer by that belt, so you could size them in std. 30-06 reloading dies. You might even specify the original 30-06 extraction rim, so that the belt stands proud, from both the case body and the rim. This way, any 30-06 chamber could be modified by a cutter putting in the belt recess, and then headspacing off of this new belt. Now the case shoulders, and chambers, need to be reformed back .030" so that original military brass won't chamber anymore, but these will run through box magazines, and give usable bores right up to .416 caliber. I recall reading about someone playing with the very similar 375 H&H intermediate magnum, to do the same thing. But, IMO, this would be a killer for the 400 Whelans, out there. Even if the shoulders of the factory parallel rounds are shortened some .030", the case mouths O.A.L.'s could still be at 30-03+ lengths. Utilizing the standing proud, idea, you wouldn't have to mess with the rifle's extraction, and once you got above 35 caliber, you wouldn't have any commercial cases to generate conflicts. I believe .416 is about the max for this idea, but there are now a bevy of 330 and 350 gr. bullets for this bore. The 405 Win, and this rimless belted counterpart, would both use the same round nosed .412 diameter bullets. Down at the .375 level, the 235 to 270 gr. slugs would make this sub magnum, shine. Since posters on this forum have noted that to do Ackley improved chambers correctly, you have to take the barrel off and short chamber so the Go gauge, becomes the No Go. So then, doing the same thing, and just reaming the belt recess, and longer neck-throats, would seem to be about equal for the gunsmith's time. Setting the barrel back a thread, would still be the same job, more or less. Just a different reamer, and no change in powder capacities. You'd have a little longer case, but with the same capacity as the parent, if your neck increase precisely equals the shoulder's setback, volumetrically. Your 400 belted Whelan case, would be a bit longer, say at 30-03 length. Seating the bullet deeper to get the original O.A.L., would bring things right back to the original lengths, if not the original capacities, which would require the longer length necks. This would mean you could recut any Win. M95 barrel or Browning long action lever, to .405 and chamber the stand proud belted number, without messing with the big levers action's rimless breech bolts, and still have really positive head spacing. But I will draw the line with trying this out on the various gas powered auto loaders, or the really light Remington pumps. The H&H belts were supposed to work through break actions and box mags equally well. I only hope this adds rather than subtracts from this thread.